As has been stated ---the 45 ACP (Automatic Colts Pistol) cartridge
didn't exist until after 1900, so no antique.
Depending on where you live, you may be able to get black powder
firearms delivered directly to you. Either single shot or revolvers, they
load with loose powder and round or conical bullets.
I'm guessing that you just want something that LOOKS like an old pistol,
and that 3 minutes ago you had no idea what ACP stood for.
Welcome to the forum.